Charge transporting in an electrophotographic light-sensitive material involves the steps of:
(1) Generating an electric charge by light exposure and
(2) transporting the electric charge. A selenium light-sensitive plate is an example of a material capable of performing steps (1) and (2). Steps (1) and (2) may be performed by different materials, such as the well known combination of amorphous selenium and poly-N-vinylcarbazole. Systems which perform steps (1) and (2) utilizing different materials is advantageous in that the range of selecting materials used for light-sensitive materials can be enlarged. Accordingly, the electrophotographic characteristics such as the sensitivity of the light-sensitive materials, the receptive potential of the light-sensitive materials, etc., are improved. Furthermore, materials suitable for making light-sensitive coatings or layers can be selected over a wide range.
Examples of photoconductive materials for light-sensitive materials usually used in electrophotographic systems, include inorganic materials such as selenium, cadmium sulfide and zinc oxide.
U.S. Pat. No. 2,297,691 to Carlson, discloses an electrophotographic material composed of a support having coated thereon a material which shows an insulating property in the dark and changes its electric resistance in accordance with the irradiated amount of light during image exposure. After being subjected to a dark adaptation for a proper period of time, the photoconductive material is generally uniformly charged, on the surface, in the dark. Then, the photoconductive material is image-exposed to a pattern of light irradiation. The irradiation reduces the surface charge in proportion of the relative energy included in the various portions of the pattern of light irradiation. The surface charge remaining on the surface of the layer of the photoconductive material (light-sensitive layer) or the electrostatic latent image thus formed on the surface is converted into a visible image by contacting the surface thereof with a proper electroscopic material, i.e., a toner. The toner can be attached onto the surface of the light-sensitive layer in proportion to the charge pattern in both the state contained in an insulating liquid and the state contained in a dry carrier. The toner image thus formed can be fixed by a known means such as heating, pressing, or a solvent vapor. The toner can be alternatively transferred onto a secondary support (e.g., a paper, film, etc.,) and then fixed. The foregoing electrostatic latent image can be transferred onto a secondary support and developed. An electrophotographic process is an image-forming process which involves forming images in a manner such as described above.
Light-sensitive materials used in such electrophotographic processes must have such fumdamental characteristics as: (1) light-sensitive material can be charged at a proper electric potential in the dark, (2) it lets escape less of the electric charge charged on the surface thereof in the dark, and (3) it discharges quickly by the irradiation of light. Inorganic materials generally used for photoconductive materials have various advantages and disadvantages. Selenium which is widely used at present sufficiently satisfies the above-mentioned factors (1)-(3). However, production costs are high, and the material has poor flexibility, making it difficult to form a belt from such a material. The material must be handled with great care since it is very sensitive to heat and mechanical impact. Cadmium sulfide and zinc oxide are also used as a light-sensitive materials which are dispersed in a resin binder. However, such a light-sensitive material is poor in such properties as smoothness, hardness, tensile strength, abrasion resistance, etc.
Recently, electrophotographic light-sensitive materials using various organic materials have been proposed for eliminating the disadvantages of inorganic materials. Such materials have been practically used. For example, there is a light-sensitive material composed of poly-N-vinylcarbazole and 2,4,7-trinitrofluoren-9-one (see, U.S. Pat. No. 3,484,237), poly-N-vinylcarbazole sensitized by a pyrylium salt series dye (see, Japanese Patent Publication No. 25,658/'73), a light-sensitive material composed of an eutectic complex of a dye and a resin as the main component (see, Japanese Patent Application (OPI) No. 10,735/'72), etc.
Other recently proposed materials include: a high-sensitive electrophotographic light-sensitive material composed of a combination of a material generating an electric charge by the action of light (hereinafter, referred to as a charge generating material) and a material capable of transporting the charge thus generated (hereinafter, referred to as a charge transporting material). For example, U.S. Pat. No. 3,791,826 discloses a light-sensitive material composed of a charge generating layer and a charge transporting layer formed thereon. U.S. Pat. No. 3,573,906 discloses a light-sensitive material composed of a charge transporting layer and a charge generating layer formed thereon. U.S. Pat. No. 3,764,315 discloses a light-sensitive material having a light-sensitive layer formed by dispersing a charge generating material in a charge transporting material. In the light-sensitive materials of this type, many useful charge generating materials have been proposed. However, no practical useful charge transporting materials have been proposed at present. An excellent charge transporting material can sufficiently transmit light of wave length capable of generating a charge from a charge generating material to the charge generating material. The material, when charged, can maintain sufficient potential, and the material can quickly transport the charge generated by a charge generating material.